This invention relates to a submarine optical fiber cable which is suitable for use as an optical fiber cable laid at relatively shallow depths below the surface of the sea.
In general, submarine optical fiber cables are designed to exhibit mechanical properties whereby the tensile strength is in excess of 10 ton.f and the cable modulus (cable breaking tension versus cable weight in water) exceeds 20 km so that the cable can be laid to depths of 5000 m or more and recovery and maintenance work on the cable can be performed.
A conventional submarine optical fiber cable of the type described above is shown in FIGS. 2 and 3.
In FIG. 2, reference numeral 1 represents an optical fiber unit in which a plurality of core optical fibers are included, iron segments 2 that divide the circumference of the optical fiber unit 1 into three pieces are provided around the same, and a plurality of steel wires 3 are wound in a close fitting manner around the outer surfaces of the iron segments 2 that divide the circumference of the optical fiber unit 1 into three pieces. Reference numeral 4 represents a copper tube which is formed in such a manner as to surround the circumference of the steel wires 3. Reference numeral 5 represents a sheath.
In this conventional example, two layered structures formed by the steel wires 3 and the iron segments 2 that divide the circumference of the optical fiber unit 1 into three pieces together improve the pressure resistance and the tensile strength.
In FIG. 3, three different diameter steel wires 12a, 12b and 12c are wound around the optical fiber unit 11 in a close fitting manner to form two layers. Provided around this is a copper tube 13 and a sheath 14.
In this case too the two-layered tension body is formed by the steel wires 12a, 12b and 12c.
In the above-described conventional examples, the dimensions and materials thereof are, of course, designed to obtain the mechanical characteristics described above.
A submarine optical fiber cable with the same type of structure is conventionally used at any depth below the sea surface. However, since such an optical fiber cable is usually designed to be able to withstand conditions at a depth of 5000 m or more, the performance exceeds by a considerable extent what is required in a case where the cable is laid at a shallow depth and use of such expensive cable in such location is thus uneconomical. Furthermore, since a cable of the type described above has a relatively large diameter, a great quantity of armoring materials need to be used if armoring is required. The large diameter and heaviness of the cable also lead to costly installation and transportation budgets.
For the above-described reasons, an economical submarine optical fiber cable for use at a shallow depth below the sea surface is required.
One example of such a structure is formed in such a manner that the iron segments 2, dividing the circumference of the optical unit 1 into three pieces, are omitted from the cable structure shown in FIG. 2, and another example involves omitting from the cable structure shown in FIG. 3, the steel wires 12b and 12c that form the second layer.
However, a cable with only one layer of steel wire wound directly around the optical fiber unit displays a very unstable stranded structure. Specifically, if the optical fiber unit should happen to be directly subjected to a lateral pressure during manufacture, the strength of the core optical fibers would be reduced. Consequently, a reliable submarine optical fiber cable cannot be obtained in this way.
On the other hand, another method could be employed in which the diameter of the cable is reduced by adopting smaller dimensions compared with the conventional cable structure. However, this would also lead to the diameter of the optical fiber unit being reduced. This raises problems in the sense that the number of optical fibers to be installed is necessarily limited and the mechanical properties of the optical fiber unit cannot be maintained at the required level.